In the chemical industry there are many known reactions, such as alkylation, esterification and isomerization, etc., requiring an acid catalyst. Up to this time, such reactions have been conducted by the use of acid catalysts such as sulfuric acid, aluminum chloride, hydrogen fluoride, phosphoric acid and p-toluenesulfonic acid. However, these acid catalysts have the property of corroding metals, which make it indispensable to use an expensive corrosion-resistant material or to apply an anticorrosion treatment. Further, the acid catalysts have difficulty in separation from the reaction mixture and also had such a significant environmental problem that the use of the catalysts was indispensably accompanied with waste acid disposal, which involved alkali cleaning and other troublesome steps. Additionally, the catalysts were very difficult to re-use.
Solid acid catalysts containing a sulfate group have been proposed in order to solve the above problems, the catalysts being produced by bringing a hydroxide or hydrated oxide of a Group IV metal of the periodic table into contact with a solution containing a sulfureous component and calcining the hydroxide or hydrated oxide thus treated at 350 to 800.degree. C. (Japanese Patent Publication No. 59-6181). These solid acid catalysts exhibit an acid strength higher than that of 100% sulfuric acid (Hammett acidity function H.sub.0 : -11.93). These catalysts have high catalytic performance for various acid-catalyzed reactions by virtue of the high acid strength, and are advantageous in that the corrosiveness is low, that they are easily separable from the reaction mixture and can dispense with waste acid disposal, and that they can be re-used. Accordingly, these solid acid catalyst are expected to substitute for the acid catalysts of the prior art in various reactions.
Further, it has already been known to the public that a catalyst produced by calcining zirconia gel containing a sulfureous component and incorporating platinum into the resulting zirconia exhibits high activity in the isomerization of hydrocarbons (U.S. Pat. No. 3,032,599).
Modifications of the above process for the preparation of the metal oxide catalyst mainly used for isomerization and containing a platinum-group metal and a sulfureous component are disclosed in Japanese Patent Publication Nos. 5-29503, 5-29504, 5-29505 and 5-20506. They include a modification comprising omitting the step of calcining between the step of treatment with a sulfureous compound and the step of deposition of a platinum-group metal, another modification comprising conducting the treatment with a sulfureous compound and the deposition of a platinum-group metal in reverse order, and still another modification comprising varying the sulfureous compound to be used.
Meanwhile, it is also known that a solid acid catalyst prepared by adding a sulfureous compound to aluminum hydroxide or aluminum oxide and calcining the obtained mixture exhibits an acid strength higher than that of 100% sulfuric acid (Japanese Patent Laid-Open No. 5-96171, and Arata, "Trends in Physical Chemistry", Vol.2, Section 1 (1991)).
Japanese Patent Publication No. 6-29199 discloses an isomerization catalyst prepared by incorporating both of at least one metal selected from among Ni, Ru, Rh, Pd and Pt and a sulfate group into a hydroxide or oxide of a Group IV metal selected from among Si, Ti, Zr and Sn and/or a hydroxide or oxide of aluminum. Further, a catalyst comprising a Zr(OH).sub.4 /Al(OH).sub.3 powder produced by coprecipitation and both of platinum and sulfuric acid supported on the powder is disclosed in Example 6 of this patent.